This application is based on applications Nos. H11-132202, H11-132203, H11-132204, and H11-132205 all filed in Japan on May 13, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a diffuser as exemplified by a focusing screen designed for use in a camera, for example a focusing screen incorporated in a viewfinder of a single-lens reflex camera, or a diffusive screen suitable as a screen for image projection.
2. Description of the Prior Art
The viewfinder of a common single-lens reflex camera is so configured that the light introduced through a taking lens is reflected from a main mirror in such a way as to form an image on a focusing screen having a light-diffusing function. The focusing screen is disposed at a position optically equivalent to the film surface, and therefore, by observing, through a pentagonal prism or eyepiece lens, the focus condition of the viewfinder screen formed on the focusing screen, it is possible to check the focus condition of the image that will be formed on the film surface.
The brightness and the degree of blurred appearance on the viewfinder screen depend on the diffusion angle with which the focusing screen diffuses light (i.e. how the focusing screen spreads light). Conventionally, a focusing screen is known that utilizes diffraction to diffuse light. In this type of focusing screen, diffraction is achieved by an array structure having one to several types of microstructures arranged in a pattern repeated with a pitch of about 20 xcexcm (for example a microlens array structure, or an array structure having a plurality of minute conical, polygonal-pyramid-shaped, or otherwise shaped microprisms arranged in an array). Here, the diffusion angle can be controlled by how the microstructures forming the array structure are shaped and how the pattern thereof is repeated, and by controlling the diffusion angle in this way, it is possible to increase the brightness and simultaneously the degree of blurred appearance on the viewfinder screen. Increasing the degree of blurred appearance helps exaggerate blurred appearance when an image being observed is out of focus, and thereby makes it easy to check the focus condition.
However, a focusing screen having an array structure as described above does not diffuse light evenly, and thus does not offer natural blurred appearance. That is, this type of focusing screen suffers from unevenly bright blurred appearance (such as two-line blurred appearance). A focusing screen having a surface like that of frosted glass produces densely diffused light, and thus offers natural blurred appearance. However, this type of focusing screen diffuses light with too large a diffusion angle, with the result that the amount of light reaching the observer reduces quickly, making the viewfinder screen appear dim.
FIG. 7 shows an example of a conventionally well-known focusing screen. This focusing screen has a Fresnel lens surface (S1) formed on its side facing a taking lens (not shown), and has a diffusive surface (S2) formed on its side facing an eyepiece lens (not shown). The Fresnel lens surface (S1) has an optical power that tends to direct the off-axial rays incident on the viewfinder screen at the very edges thereof to the pupil (in other words, this surface has a light-condensing function). The light (L0) from a subject introduced through the taking lens is deflected by the Fresnel lens surface (S1) so as to form an image on the diffusive surface (S2), and then travels further, as diffused light, toward the eyepiece lens. Part of the diffusive surface (S2) is formed into a display surface (S2a), which is treated with a reflection enhancement process. This display surface (S2a) corresponds to display presented within the viewfinder screen as, for example, an AF (autofocus) frame. When the display surface (S2a) is illuminated with illumination light (L1), it acts as a prism to reflect the illumination light (L1) toward the eyepiece lens.
As described above, a conventional focusing screen typically achieves display by being illuminated at a display surface (S2a) formed therein. However, this is possible only when the focusing screen is treated partially with a reflection enhancement process to form a reflecting surface that serves as the display surface (S2a).
FIG. 36 shows another example of a conventionally known focusing screen. This focusing screen has a Fresnel lens surface (S1) formed on its side facing a taking lens (not shown), and has a diffusive surface (S2) formed on its side facing an eyepiece lens (not shown). The symbol AX represents the optical axis of the Fresnel lens surface (S1). The Fresnel lens surface (S1) has a light-condensing power that tends to direct the off-axial rays incident on the viewfinder screen at the very edges thereof to the pupil (so as to match pupils). The light introduced through the taking lens is deflected by the Fresnel lens surface (S1) so as to form an image on the diffusive surface (S2). This type of focusing screen structure is generally known. For example, Japanese Patent Application Laid-Open No. H8-129205 proposes a diffusive screen having on one side a diffusive surface (S2) formed by superimposing a directional pattern and a non-directional pattern on each other and having on the opposite side a Fresnel lens surface (S1).
In a case where, as shown in FIG. 36, the Fresnel lens surface (S1) is located away from the diffusive surface (S2), the imaging performance on the diffusive surface (S2) depends on the imaging performance of both the taking lens and the Fresnel lens surface (S1) considered together as a composite optical system. Thus, the Fresnel lens surface (S1) is one of the factors that degrade the imaging performance (for example aberration characteristics) of the taking lens. Bringing the Fresnel lens surface (S1) close to the diffusive surface (S2) or forming the Fresnel lens surface (S1) on the diffusive surface (S2) causes the zonal fringes (i.e. concentric circular lines) of the Fresnel lens surface (S1) to be observed through the eyepiece lens. This, too, degrades the imaging performance. On the other hand, locating the Fresnel lens surface (S1) farther away from the diffusive surface (S2) not only degrades the aberration characteristics and other performance mentioned above, but also causes vignetting that reduces the amount of light reaching the edge portion of the viewfinder screen and thus makes it appear dim.
An object of the present invention is to provide a focusing screen that offers a satisfactorily high degree of blurred appearance with even brightness and that offers a bright screen.
To achieve the above object, according to one aspect of the present invention, a focusing screen for use in a camera is provided with a diffusive surface having a plurality of random pattern cells of one or a plurality of types arranged two-dimensionally so as to form a macroscopically flat surface. Here, the random pattern cells are each composed of a plurality of minute rectangular elements having one of two to eight types of heights. Moreover, the individual rectangular elements are arranged irregularly so as to constitute a plurality of types of diffraction gratings having different grating pitches.
According to another aspect of the present invention, a focusing screen for use in a camera is provided with a diffusive surface having a plurality of random pattern cells of one or a plurality of types arranged two-dimensionally so as to form a macroscopically flat surface. Here, the random pattern cells are each composed of a plurality of minute rectangular elements having one of three or more types of heights. Moreover, the individual rectangular elements are arranged irregularly so as to constitute a plurality of types of diffraction gratings having different grating pitches. Furthermore, the rectangular elements each have their top-end surfaces formed into a convex shape.
According to another aspect of the present invention, a focusing screen for use in a camera is provided with a diffusive surface having a plurality of random pattern cells of one or a plurality of types arranged two-dimensionally so as to form a macroscopically flat surface. Here, the random pattern cells are each composed of a plurality of minute rectangular elements having one of three or more types of heights. Moreover, the individual rectangular elements are arranged irregularly so as to constitute a plurality of types of diffraction gratings having different grating pitches. Furthermore, this focusing screen is further provided with a display surface that is disposed on the identical plane on which the diffusive surface is disposed and that is formed as a diffraction grating configured so as to have regular periodic structures.
According to another aspect of the present invention, a focusing screen for use in a camera is provided with a diffusive surface having a plurality of random pattern cells of a plurality of types arranged two-dimensionally so as to form a macroscopically flat surface. Here, the random pattern cells are each composed of a plurality of minute rectangular elements having one of two or more types of heights. Moreover, the individual rectangular elements are arranged irregularly so as to constitute a plurality of types of diffraction gratings having different grating pitches. Furthermore, the rectangular elements have an increasingly large proportion of high-frequency components from the center to the edge of the focusing screen.
According to another aspect of the present invention, a diffuser is provided with a diffusive surface having a plurality of random pattern cells of one or a plurality of types arranged two-dimensionally so as to form a macroscopically flat surface. Here, the random pattern cells are each composed of a plurality of minute rectangular elements having one of three or more types of heights. Moreover, the individual rectangular elements are arranged irregularly so as to constitute a plurality of types of diffraction gratings having different grating pitches. Furthermore, the rectangular elements each have their top-end surfaces formed into a convex shape.